金属有机骨架化合物的二氧化碳吸附性能的研究进展

Research Progress on Improving the Capture of Carbon Dioxide by Metal-Organic Frameworks

能源与环境是人类生存和发展的必要条件,两者协调发展是社会实现可持续发展的重要保证。近几年来,人类赖以生存的化石燃料所带来的负面影响逐渐受到社会的关注,而化石燃料燃烧所释放出的CO2是造成温室效应的主要原因。因此在低碳经济环境下开发出高效环保的碳捕获和封存技术,对能源循环利用及环境保护起到至关重要的作用。使用胺溶液进行洗涤及吸收CO2是工业上捕获和存储(CCS)最常用的技术之一(例如从电厂烟气中分离CO2),该法可以大幅减少CO2的排放量,但同时也会增大工厂的能源消耗(25%～40%),从而大幅增加额外成本。胺洗涤的其他缺点包括碱溶液对设备的腐蚀、溶剂的损失、大量产热引起的胺降解以及捕获之后不易被分离出来。另一种捕获CO2的方法是采用高温下吸收(化学吸附)的固体材料如碱金属陶瓷、固体胺、层状双氢氧化物或钙基吸附剂,但此类方法的能量消耗和对水分子及其他组分的敏感性限制了其应用范围。此外,采用聚合物或无机膜,在不同的机制下选择性分离混合气体也是一种可行的方法,但很难获得具有高稳定性、高选择性和高通量的薄膜,并且提高膜的吸附分离作用和选择性非常必要。对于固体吸附剂而言,高压下多孔材料对CO2的捕获是以吸附剂与被吸附物相互作用为主,而在低压或低CO2浓度下的选择性捕获主要受吸附剂与被吸附物相互作用以及吸附剂对CO2的化学亲和力两者共同影响。金属有机骨架化合物(MOFs)具有高结晶度、高比表面积和可调的孔隙结构,在气体吸附尤其是CO2捕获方面展示出巨大的潜力。相对于活性炭、沸石等固体吸附剂来说,MOFs具有更高的吸附选择性。将其应用于碳捕获和封存技术中,可以大幅拓宽CO2吸附剂的可选择范围,在提高吸附选择性的同时,也可以有效地降低成本。目前,有望采用MOFs材料捕获CO2的场合包括发电厂的碳捕集、天然气中CH4/CO2的分离、交通工具排放的CO2的收集甚至直接从空气中捕获。因此,研发能够高效吸附分离CO2的MOFs材料对于缓解环境压力意义重大。本文概括了CO2吸附模型的建立方法,提出了几种提高MOFs对CO2捕获量的策略。如提高开放金属位点的密度、掺杂金属或氮原子、调节孔径或进行氨基功能化以及合成MOFs复合材料等,并比较了不同方法对于改善低压条件下CO2吸附量的影响,有望将其应用于捕获燃烧后烟道气、汽车尾气以及其他小型排放源中的CO2。

Energy and environment are the essential conditions for human survival and development,furthermore,the mutual coordination between them is a vital guarantee for social sustainable development.In recent years,the negative impact of fossil fuels on human survival has gra-dually attracted widespread attention from society.The greenhouse effect is mainly attributed to the release of CO 2 from the burning of fossil fuel.Therefore,the development of efficient and environmental-friendly carbon capture and storage technologies in a low-carbon economy environment play a crucial role in energy recycling and environmental protection.Utilizing the amine solutions for scrubbing and absorbing CO 2 is one of the most commonly technologies for industrial capture and storage(CCS)(e.g.,separating CO 2 from flue gas of power plant flue gas),which can significantly reduce the CO 2 emissions,but also increases the plant energy consumption by 25%~40%,hence leading to an increase in additional costs to a large extent.In addition,other disadvantages of amine scrubbing include corrosion of the equipment by the alkaline solution,loss of solvent,degradation of the amine caused by heat production,and difficulty of separation after capturing.Solid materials such as alkali metal ceramics,solid amines,layered double hydroxides or calcium-based adsorbents for high temperature absorption(chemisorption)are another method of capturing CO 2,while the energy consumption and the sensitivity of water molecules and other components limit their scope of application.In addition,it is also a feasible method to selectively separate the mixed gases with different mechanisms by utilizing polymers or inorganic membranes,yet the membranes with high stability,high selectivity,and high throughput are hard to obtain,and it is necessary to ameliorate the membranes’adsorption and separation and their selectivity.For solid adsorbents,the capture of CO 2 by porous materials at high pressure is dominated by adsorptive interactions,while selective capture at low pressure or low CO 2 concentrations is primarily influenced by the interaction of adsorbents and a chemical affinity to CO 2.Metal organic frameworks(MOFs)exhibit tremendous potential for gas adsorption,especially for CO 2 capture,due to their high crystallinity,high specific surface area and tunable pore structure.Compared with other solid adsorbents,such as activated carbon,zeolites,MOFs have higher adsorption selectivity.Applying it to the carbon capture and storage technology can dramatically broaden the range of CO 2 adsorbents,increase the adsorption selectivity,meanwhile,effectively reduce the costs.Currently,MOFs are expected to capture CO 2 in power plants,separation of CH 4/CO 2 in natural gas,CO 2 collection from vehicles,and even direct capture from the air.Therefore,the development of MOFs mate-rials capable of efficiently adsorbing and separating CO 2 is of great significance for relieving environmental stress.This paper summarizes the establishment of CO 2 adsorption model and proposes several methods to improve the adsorption capacity of CO 2,such as increasing the density of open metal sites,doping metal or nitrogen atoms,adjusting their pore size or amino-functionalization,and synthesizing MOFs composite materials,and compared the effects of different methods on the adsorption capacity of CO 2 under low pressure.In addition,they are expected to be applied to the capture of CO 2 in post-combustion flue gas,vehicle exhaust and other small emission sources.